The present invention relates to novel molding compositions based on semicrystalline engineering thermoplastics, which give laser-markable moldings with increased quality of marking. The invention further relates to moldings which are produced from these laser-markable molding compositions, to a process for laser-marking, and also to the use of selected additives for laser-marking.
Semicrystalline thermoplastics are materials with a long history of use. Functionalities such as ability for marking by laser light are of increasing significance, alongside the mechanical, thermal, electrical, and chemical properties of these materials. Examples that may be mentioned are applications in the household products sector, in keyboards, and in the electronics sector. Here, the application demands high contrast between the laser-inscribed marking and the background polymer matrix. Other advantageous factors for aesthetics are precise setting of the color of the marking and also of the matrix, and the nature of the surface of the marking. Especially if the use of the laser-marked surfaces usually renders them visible or regularly exposes them to mechanical or chemical stresses, there is a need to provide durability together with high perceived quality. In many cases this inhibits use of semicrystalline engineering thermoplastics.
Laser-markable molding compositions are known per se. One method for marking of these molding compositions with laser light consists in irradiating selected sites on the plastic with laser light and using the energy introduced to bring about a mechanical alteration or a local discoloration of the plastic. Another method for marking these molding compositions with laser light consists in using discolorable fillers, which change their color via irradiation by laser light at selected sites.
EP-A-190,997 discloses a process for laser-marking of pigmented systems, by incorporating, into a high-molecular-weight material, an additive which is sensitive to radiation and which causes discoloration. Among the additives mentioned are TiO2 and Sb2O3.
The use of a very wide variety of additives in laser-markable plastics molding compositions is described in a very wide variety of patent documents. For example, EP-A-330,869 discloses the use of TiO2 white pigment. EP-A-400,305 and EP-A-542,115 describe the use of copper hydroxide phosphate and, respectively, molybdenum(IV) oxide.
U.S. Pat. No. 5,063,137 discloses inter alia the use of anhydrous metal phosphates and phosphoric-acid-based glasses.
EP-A-797,511 describes the use of lamellar pigments with a layer composed of doped tin oxide.
U.S. Pat. No. 5,489,639 describes the use of selected copper salts (phosphates, sulfates, thiocyanates).
EP-A-764,683 discloses the use of copper pyrophosphate hydrate and/or manganese sulfate hydrate.
EP-A-808,866 discloses the use of boric anhydride.
WO-A-98/58805 discloses a number of copper phosphates.
WO-A-99/55773 describes inter alia the use of zinc hydroxystannate and stannous oxalate.
DE-A-199 05 358 discloses the use of alkali metal copper diphosphate.
WO-A-01/007 19 describes the use of Sb2O3 with particle sizes above 0.5 μm.
EP-A-1,190,988 discloses the use of certain bismuth mixed oxides.
WO-A-01/78994 proposes the use of copper fumarate, copper maleate, and mixtures of these.
DE-A-100 53 639 proposes the use of selected salts, inter alia various cobalt phosphates and iron phosphates.
DE-A-100 34 472 describes the use of particles surface-modified with certain silicon compounds.
EP-A-753,536 describes the use of at least two types of metal oxides.
EP-A-105,451 describes polyphenylene sulfide molding compositions which have been modified for laser-marking with selected additives, for example with the combination of lead chromate and lead molybdate, with nickel antimony titanate, or with cobalt zinc silicon.
The laser-markable plastics molding compositions available hitherto are in many respects still unsatisfactory.
Pigments used for laser inscribability frequently give only an inadequate color difference between the marked site and the unirradiated plastics matrix, therefore giving poor color contrast and poor legibility of the inscription. In other cases, the color contrast which is initially acceptable alters with time, with the result that the inscription becomes almost invisible. There is also the risk of undesired alteration of the properties of the plastic as increasing amounts of pigment are added.
If a light-scattering particle is used as laser-sensitive additive, and acts as a white pigment, this approach has the resultant limitation that although increasing content of additive increases sensitivity to light it can both reduce the contrast of the marking and reduce the depth of penetration of the marking.
If a metal compound (e.g. oxide or salt) is used, the respective intrinsic color generally contributes to discoloration of the matrix and is generally detrimental to aesthetics.
Starting from the abovementioned prior art, an object underlying the present invention is to provide molding compositions based on semicrystalline engineering thermoplastics which can be marked by conventional lasers, and in which the color of the matrix, the color of the marking, and also the surface topology of the marking, can be optimized, with good contrast of the marking, without any resultant disproportionate impairment of the remainder of the property profile of the engineering thermoplastics.